Holographic filters have been used for many years. Conventional multi-line filters currently have large spectral bandwidth (typically greater than 10 nm) and a limited number of bands (typically fewer than 5). The large bandwidth is insufficient to resolve narrow spectral features in many applications, such as spectroscopy, spectral imaging and astronomical applications. The limited number of bands imposes limitations on the performance of conventional filters.
The prior art in the field of holographic filters is described in numerous printed publications and issued patents. A review of selected prior art literature including descriptions of methods of preparing and using holographic filters, and some applications of prior art holographic filters, is now presented in brief overview.
The top portion of FIG. 1(A) illustrates a prior art method of fabricating a holographic filter using the so-called reflection geometry. The prism allows steep angles to be coupled in the medium, which can generate filters in the infrared wavelength range of interest. The glass-glass interface (or other interfaces with similar refractive indices) between the prism and the holographic material can be filled with a commercially available matching fluid to eliminate reflections at the interface between the two solids. In FIG. 1, vector kr denotes a propagation vector of a coherent beam of light that is sometimes termed a reference beam, and vector ks denotes a propagation vector of a coherent beam of light that may include information that is sometimes termed a signal beam. In some instances, the two beams are coherent beams originating from the same source. The bottom portion of FIG. 1(A) illustrates the read-out of the holographic filter using a infrared probe beam having a propagation vector kIR and the resulting reflected beams having various propagation vectors kd. The diagrams shown in FIG. 1(B) illustrate the vectorial relationships that exist among the propagation vectors.
A multi-band spectral filter is fabricated by superimposing many narrow band filters with different central wavelengths in the same volume. The fabricated filter is used at almost normal incidence. The wavelengths of the incident beam that match the periods of the Bragg grating recorded in the holographic filters are reflected and therefore filtered from the incident beam. The thickness of the recorded material is limited by absorption of the recorded beam. All holographic materials have absorption at the recorded wavelength. In most instances, the thickness is limited to a couple of millimeters.
An example of a multi-line holographic filter useful for identifying a specific material based on a precise match between the spectral lines characteristic of the material and spectral bands built into the filter is described in U.S. Pat. No. 6,934,060 to Psaltis, which patent is assigned to the assignee of the present application.
U.S. Pat. No. 5,491,570 to Rakuljic at al. describes methods of writing and reading reflective plane holographic gratings that are matched at the Bragg condition to infrared radiation. The patent describes writing the gratings using either transmission mode, in which two incident beams impinge on the same face, or reflective mode, in which two incident beams impinge on opposite faces, of a crystal specimen of lithium niobate (LiNbO3) possibly doped with iron (e.g., 0.05% Fe:LiNbO3). Anti-reflection coatings can be applied to the crystal surface to reduce reflection losses and to improve the efficiency of the grating.
U.S. Pat. No. 5,335,098 to Leyva et al. describes some of the physics underlying the generation of holographic gratings in various materials, such as photorefractive materials. The patent describes and claims methods of developing and fixing holographic gratings that rely on the application of electric fields and thermal treatments, simultaneously or in sequence. The patent also claims filters for reflecting a single band, and mentions wavelength multiplexed holograms. U.S. Pat. No. 5,440,669 to Rakuljic et al., which matured from the parent application of the application that earlier matured into U.S. Pat. No. 5,335,098, also discusses methods of making and uses of holographic recordings. Each of U.S. Pat. Nos. 5,335,098, 5,440,669, 5,491,570, and 6,934,060 is incorporated by reference in its entirety herein.
There is a need for holographic filters that provide both a plurality of narrow lines and offer the possibility of large effective apertures.